Support for group14, group16 Kex and HMAC EtM

[edgsousa]

Hi

Some devices I work with were upgraded to a ssh config that only allows kex group14-sha256. Also I saw the request #1229, and decided to implement that as well.

[fuhry]

For anyone who would like an unofficial build with both curve25519+sha256@libssh.org kex support and @edgsousa's EtM patch, I've put those together into a branch here: https://github.com/fuhry/paramiko/tree/curve25519%2Betm

[mikeln]

LGTM and it appears to fix several of my issues with SSH to Azure nodes.

[mikeln]

How do we get this PR reviewed , merged, and added to a release? I.e. who is the person we need to bother?

[ploxiln]

for what it's worth, I've started a fork, merged this and a few other things according to my own taste, and made an initial release: https://github.com/ploxiln/paramiko-ng/releases/tag/2.5.0

[bitprophet]

Merged to latest master, cleaned up some code a bit, fixed a copyright header (I hate you, LGPL) and added a changelog. Then tested against OpenSSH and both new MACs and both new kexen seem to work great. Thanks!